Obturation , PRB and rifling

[Vaino]

Obturation is the complete sealing of the bore to prevent gas from escaping past the projectile. The patch accomplishes this if properly sized to the RB and the bore. The small amount of RB compression and the resulting small amount of dia. increase is not obturation.

If the patch is too thin, the gas leakage will be in the grooves of the rifling....the RB and patch are squeezed much more tightly between the lands, so leakage is unlikely to occur there.

As an example...if the bore {land dia.} is .540 and the grooves are .015 deep, the groove dia. is .570. If a .530 dia. RB is used w/ an .018 thick patch, obturation will not occur because this combo only adds up to .566 and is .002 short per side of filling the groove and therefore no obturation takes place. A .020 thick patch would just fill the groove, but obturation might also not occur. .022 and .025 patch thicknesses definitely do overfill the groove and therefore obturation would occur. In my .54., .535 dia. RBs and a patch thickness of .022 is prefered.{.579 total}

Lately have been using round bottom rifling and think there's an advantage in obturating the bore VS flat bottom rifling. The rounded groove requires a smaller volume of patch mat'l to fill it and w/ the proper patch thickness, obturation would be erasier to achieve. Also the corners of flat bottom rifling would be harder for the patch to fill.

These are my thoughts on this subject and would be interested to hear other opinions....Fred

 

[Rifleman1776]

I doubted the correctness of your definition. So, to prove you wrong I looked up the word "obturation". Yer right. :redface:
I had long believed the word referred to the squishing/squashing/distortion of the lead ball due to the compressive force of the powder explosion.
Wrong.
So, your post is accurate and demonstrates the need for a proper ball/patch combo.
OK, ye dun gud this time. :applause:

 

[Vaino]

The lengthwise compression and resultant increase in dia of a conical would cause obturation if the conical is the correct size. How much compression takes place, I don't know, but it's not very much. Actually a conical of the correct size, possibly obturates the bore better than a PRB. It would be interesting to find out how much gas escapes {if any} past the patch mat'l in the groove w/ various combos of patch thicknesses and RB dias.....Fred

 

[Larry Pletcher]

Since you mention conicals, In the long range muzzleloading world, a round grease groove bullet fired in a hex rifled bore is recovered with a hex shape.

In my .45 badger barreled long range barrel, a paper patch bullet that loads with the weight of the ramrod, slices the paper patch when fired. These bullets have rifling marks on them when recovered. If fired with a mild load, the paper patch burns instead - burned by gases passing the bullet. In this gun, obturation is based on the strength of the load.

Both of these examples "bump up"; whether complete obturation takes place, I don't know.

regards,
Pletch

 

[Greenjoytj]

I believe the concept of achieving complete 100 % obturation in a muzzleloader shooting a PRB is a myth.

You can get a good gas seal but it will never be 100%, some gas always gets past the PRB on its way to the muzzle.

Round bottom rifling may improve the ability of the patch to seal the to the rifling grooves but even that won't be a 100% seal.

The first shot from a clean bore, if the recovered patch had no star pattern of fouling from the high pressure gas passing the patch then I would believe the obturation is close to 100% at that load pressure.

With out precise laboratory testing the shooter will never know how close to a 100% seal his ball and patch combo is.

Changing the powder charge which will change the chamber pressure may make a good seal bad blowing your groups apart. I think this is why so many shooter report good groups with medium size powder charges but get poor groups with heavier powder charges. In this case the internal pressure has exceeded the ball and patch ability to seal the grooves and too much gas is escaping past the patch blowing the group. A larger ball with a properly filled patch is required to restore accuracy.

In my rifle the powder charge, ball size, patch thickness and lub, OP wad size and lub are providing me with amazing accuracy. But I don't for a minute consider the load achieving 100% obturation but it is close enough to help provide the accuracy I am getting.

 

[Vaino]

Your post is most welcome and I agree w/ all the "points" you make. Complete obturation is difficult to achieve w/ a PRB and the "weak spot" is the patch mat'l in the groove. So...why are the groove depths so deep when shallower grooves would seal better? Many bbls today have groove depths of .014-.016 and have wondered why so deep? Half that depth would yield better patch sealng in the grooves. Have shot "big" PRB loads in a TC Hawken w/ much shallower groove depths w/ excellent accuracy. Possibly harder loading which was present would be the factor for deeper grooves? I don't think the deeper grooves prevent PRB slippage, so in the end, why such deep grooves? Fred

 

[Dphar]

The lengthwise compression and resultant increase in dia of a conical would cause obturation if the conical is the correct size. How much compression takes place, I don't know, but it's not very much. Actually a conical of the correct size, possibly obturates the bore better than a PRB. It would be interesting to find out how much gas escapes {if any} past the patch mat'l in the groove w/ various combos of patch thicknesses and RB dias.....Fred

The round ball "bumps up" hardly at all.
The elongated bullet with more inertia expands rather easily with BP.
I have shot PP bullets from 40 calibers BPCRs that were .009.-.010 under groove dimension with good success.
I would not count in the typical RB expanding to the point it will seal the bore. If you look at Sam Fadala's research into sealing the bore with a patched round ball its very unlikely without wad behind the ball.
Its detailed on one of his books on Muzzleloading.
The photos he took invariably showed that unwadded PRBs leaked smoke and fire.
Most ML barrels have grooves that are too deep. Regardless of the groove shape grooves much over .010 are hard to seal with cloth and even .008 deep may not seal completely unless very tight. Like a .451 ball in a 45 with a substantial patch.

Dan

 

[Vaino]

Thanks for the reply... I do agree w/ the points in your post. Still the question persists...why such deep grooves and did the originals have such deep grooves? Or did they? Must be a reason if so.....Fred

 

[LRB]

Hugh Toenjes claims they did, and cuts his grooves at around .025 deep. Each barrel he cuts is tested for acuracy at 100 yrds from some sort of machine rest. Here is a pic of one target. I thought I had more pics, but I cannot find them. Before you ask, his work is on the expensive side.

 

[zimmerstutzen]

Wick, we need the rest of the story!!!

How many lands and grooves? What width ratio for the lands and grooves. (Pope preferred lands to be 1/7th the width of the grooves) Round bottom, square bottom, oval bottom, etc.

 

[zimmerstutzen]

I agree that a round ball probably is not affected the same by powder ignition as a long heavy bullet is affected.

To me, the ball should be imprinted with the weave pattern, especially where in contact with the lands.

In a gun with very wide grooves and narrow lands, the ball might even be indented slightly where the PRB contacts the lands.

 

[Vaino]

Hi Wick...I'm wondering if the superb accuracy of Toenjes bbls is caused by his precision machining. I know his bbls are too expensive for my builds.

The land dia. is .450 and if 2 x.025 groove depth is added, the groove dia. is .500. The RB's dia is .440 and if 2 x .026 patch thickness is added, the total is .492. This RB and patch combo wouldn't fill the grooves....004 short for each groove. This would lead me to believe that quite a bit of gas is escaping past the PRB.

Although possibly many shoot similar "loose" PRBs and the accuracy may be good, I wouldn't consider shooting a patch/ball combo that didn't come close to filling the grooves. Way too inefficient.

Perhaps the quoted specs are wrong or something else? Thanks for the post....Fred

 

[LRB]

I don't think the specs given are wrong, and on the face of the matter, I agree with you. However, I would say if accuracy is a serious concern, I would sacrifice a little efficiency, if that's what it took to get it. It may be that the deep grooving is much more forgiving as to exact powder loads accuracy wise. I have a friend whose gun has a Parris barrel with deep round bottom grooves. He never worked up an optimum load for it, and changes the charge depending on the distance. He sighted in at 50 yards, then experimented to find the charges for 25 yd, 75yd and 100 yd to get his POA all the same. He just chose what he thought should work at 50yd, and sighted in. He comes in 1st to 2nd place at any match he shoots in. Go figure. He has never fired a group to check the spread. I was amazed at what he can do with the gun.

 

[nkbj]

I give up. Why are the grooves in round ball barrels so deep? Maybe it's like Mardi Gras and if you don't have excess you don't have enough.

 

[Dphar]

Hi Wick...I'm wondering if the superb accuracy of Toenjes bbls is caused by his precision machining. I know his bbls are too expensive for my builds.

The land dia. is .450 and if 2 x.025 groove depth is added, the groove dia. is .500. The RB's dia is .440 and if 2 x .026 patch thickness is added, the total is .492. This RB and patch combo wouldn't fill the grooves....004 short for each groove. This would lead me to believe that quite a bit of gas is escaping past the PRB.

Although possibly many shoot similar "loose" PRBs and the accuracy may be good, I wouldn't consider shooting a patch/ball combo that didn't come close to filling the grooves. Way too inefficient.

Perhaps the quoted specs are wrong or something else? Thanks for the post....Fred

Deep grooves in themselves do not an accurate rifle make.

Jim McLemore (sleepyhillbarrels.com) makes one hole barrels too, tested before shipping. They are made of high quality certified barrel steel and have narrow lands and "square" grooves and the grooves are only .008"
He claims that these barrels virtually never wear out.
I have 2 barrels made by Jim, one 50 and one 45 1 1/4"x 44" no taper chunk rifle type barrels. Both are gain twist. I shot a 10 shot match with the 50 caliber 3 months ago and counting 2 sighters 8 of the 12 shots would have struck Nickle centered over the "X" and 6 would have struck a dime. This in the wind in competition. So you don't get to pick a nice still morning to shoot the shots.
This is a composite of the 10 shots for score. The aver deviation from center was .407"
109 gr of FFF Swiss, .495 ball, .018" patch. Schoultz method.

Rifle weight is 17 lbs. 12 ounces.
Dan

 

[Vaino]

Appreciate your thoughts, but seeing I'm mainly a hunter, just can't accept a patch/ball combo w/ such inefficiency. Granted if the goal is just accuracy, whatever "works" would be fine, but the engineer/hunter in me just can't abide w/ the "power loss" w/ "loose loads".

A flintlock loses, out of necessity, a certain amount of energy through the touch hole and w/ the "leakage" because of insufficient patch mat'l in the grooves, further energy is lost. Because of the two, a sizeable amount of the available energy is wasted. The TH loss can be kept to a minimum by a smaller dia. but it really behooves the hunter to ensure that the patch/ball dia. does obturate the bore as best it can. Loading ease for a second shot might dictate a slightly looser PRB and this should be developed. Again thanks for your reply.....Fred

 

[zimmerstutzen]

Flehto: wick didn't answer my question, but I can tell you that where the PRB contacts the rifling, it will indent the ball. It probably swells along side enough to add that .004 per groove to seal it.

For instance an old machinist/mechanics' cheap trick for a piece that is a few thousands of an inch too short. Use a prick punch and prick a dozen holes. the metal stock raises slightly like a mound around the punch holes. If you only need a few thousandths, it can do the trick on the short term.

Where that thick patch is indented into the rifling, and lead being so malleable, the excess has to go somewhere. it probably goes to the side just enough to cause the patch to fill the groove. .

Even for the old 1890's target cartridge guns, the bullet calculation was enough to fill the bore when the bullet was seated. Not bore size or groove size.

 

[paulvallandigham]

I have achieved a full seal shooting PRBs in both a .45 and .50 cal. rifle by using a filler( CoW) between the powder charge and the PRB. I have had similar results using OP wads as a firewall, although I had to buy the correctly oversized OP wads to fill the square cut grooves in the barrel.

Evidence? The LACK Of "daggers" of soot running forward from the contact ring on my spent patches. If the wrong patch and ball combination is used, gas will force its way up the grooves past the dark ring left where the patch is squeezed against the lands and grooves when seated on the powder charge.

We need to be more cautious on how we view other evidence cited as proof that gases do blow by even a well sealed projectile. In time lapse photos( NOT VIDEOS), You often will see what appears to be "Smoke" in front of a projectile leaving the muzzle. However, if you watch an actual video or film of the shot, the "smoke" is often much PALER in color, and does not take as long to disperse as does the smoke following the projectile out the muzzle. AND, unless we are shown EXACTLY how the PRB is loaded, and how the gun is clean and lubed for the shot, we simply don't know what materials are in the barrel that are pushed out by the PRB in front of it.

We do know that compressed air forms a shadow against a dark background when high lighted. We also know tat compress "humidity" will appear as steam when it is violently compressed and forced out of a tube(This can be see in looking at air rifles propelling projectiles with either air or CO2). And, if oil or grease is left in the bore by the lubed Patch, this can also be turned into a gas-like aerosol that will leave a visible shadow when high lighted against a dark back ground.

What I have seen in watching videos of PRBs leaving the muzzle is a "whitish" cloud of some gas, in front of the ball, that dissipate much quicker than does the darker smoke tat follows the projectile. Whether its compressed air, steam, or "atomized" oil or grease, or a combination of all three, matters little.

If there is no evidence on the spent patches of soot dagger running forward of the contact ring, Its NOT black powder smoke from the main charge. This is the primary reason that "reading" our spent patches is so important to working up an accurate load. A careful reading of DUTCH SCHOULTZ'S materials will bear this point out. :hatsoff:

 

[Dphar]

I give up. Why are the grooves in round ball barrels so deep? Maybe it's like Mardi Gras and if you don't have excess you don't have enough.

I think a lot of it is based on BS.
Old barrels were often cut deep to increase service life perhaps but I doubt they were .025 based on what I have seen. Recutting was a common thing but I believe it was because of bore damage to bores being left loaded with fouling in the bore from a previous shot.
If the area was even slightly hostile shooting then detail cleaning the gun before loading could be fatal.
From my reading the most they got under this situation was a quick wipe with a wad of tow or a rag with tallow on it.
I suspect that deeper grooves, maybe .015, may have worked longer between freshings. Wear/erosion was also a factor with iron barrels.
We also have to remember that many barrels were basically 7 sided with a small groove in the "corners" of the heptagon.

Dan

 

[Dphar]

Appreciate your thoughts, but seeing I'm mainly a hunter, just can't accept a patch/ball combo w/ such inefficiency. Granted if the goal is just accuracy, whatever "works" would be fine, but the engineer/hunter in me just can't abide w/ the "power loss" w/ "loose loads".

A flintlock loses, out of necessity, a certain amount of energy through the touch hole and w/ the "leakage" because of insufficient patch mat'l in the grooves, further energy is lost. Because of the two, a sizeable amount of the available energy is wasted. The TH loss can be kept to a minimum by a smaller dia. but it really behooves the hunter to ensure that the patch/ball dia. does obturate the bore as best it can. Loading ease for a second shot might dictate a slightly looser PRB and this should be developed. Again thanks for your reply.....Fred

Unless a lube such as water soluble oil that is allowed to evaporated the water away after the patches are wet is used there is no problem loading a 495 ball and a .018 patch. But it needs to have Neatsfoot oil (PURE), Mink Oil (probably Neatsfoot by a different name), tallow or neatsfoot with some beeswax melted in to form a paste at room temp so it will load OK. My 16 bore will shoot almost indefinitely with Neatfoot oil.
My 50 works pretty well for repeated shots with beef tallow that is well purified. None of these will cause rust. But the fat used to make the tallow needs 3-4 boilings in clean water to assure this.
If the barrel is a Green Mountain it will give good velocity with a -.005 ball and a patch over .015" and will be as easy to load as could be expected in a RB rifle that will give good accuracy.
Slicker lubes may require more powder to produce accuracy/low velocity variations than high friction lubes do.
I have never seen any significant obduration in round balls to at least 50 caliber. The inertia is too low, bullets expand under acceleration due to the back of the bullet moving faster than the back of the bullet causing it to foreshorten. Rifles of 66-69 caliber or larger might show this but I am not sure and have not looked at it.
I will say this ALL the PRBs I shoot show engraving of the patch material into the ball at the lands. My 16 bore (.662 ball) will show cloth print on the back of the ball as well.

This was recovered from a deer shot at the base of the throat.

Dan